1. Field of the Invention
The present invention relates to industrial X-ray tubes that are used when performing non-destructive testing for plant piping and similar structures, where electrons emitted from the cathode are caused to strike the anode, and X-rays are radiated from the anode.
2. Description of the Related Art
Conventionally known industrial X-ray tubes of such description include those having a structure in which the cathode is formed from a filament, a current is applied whereby thermal electrons are emitted from the filament, and the thermal electrons are caused to strike the anode, whereby X-rays are emitted from the anode. X-ray tubes of such description present a problem in that they are large and heavy since a filament power supply is required in addition to a high voltage power supply.
Although outside the field of X-ray tubes, electron-emitting elements in which carbon nanotubes emit electrons based on field emission are known in, e.g., the field of displays, i.e., image displaying. An electron-emitting element of such description is disclosed in, e.g., “Carbon Nanotube Field Emitter,” authored by Yahachi Saito and published in the Journal of the Surface Science Society of Japan, and JP-A 2000-090813.
Techniques in which carbon nanotubes are used to form an electron-emitting element are also known in the field of x-ray tubes. For example, such a technique is disclosed in the specifications of each of JP-A 2001-250496, JP-A 2001-266780, and U.S. Pat. No. 6,456,691. Field emission is a phenomenon in which electrons are emitted from the surface of a material when a strong electrical potential is applied to the surface of the material. Carbon nanotubes are tube-shaped particles formed from rings comprising six carbons, the particles being needle-shaped; i.e., having an extremely high aspect ratio (i.e., particle length/particle diameter).
Electron-emitting elements, in which graphite particles are used to emit electrons based on field emission, are also known in the field of displays. For example, an electron-emitting element of such description is disclosed in JP-A 2000-090813. Graphite is a layer-structured substance in which a plurality of individual carbon hexagonal planes (planes in which a plurality rings of six carbons are sequentially arranged to form a layer) are layered on each other.
Also known are electron-emitting elements, in which an end surface formed by cutting a graphite block, a carbon rod, a carbon film, or carbon fiber in a direction perpendicular to the direction in which the carbon hexagonal planes are layered. For example, an electron-emitting element of such description is disclosed in JP-A 2000-156148.
Outside the field of X-ray tubes, there are known fluorescent display devices in which the emitter portion of the cathode structure, from which the electrons are emitted (i.e., the electron-emitting portion), are structured from columnar graphite. For example, a configuration of such description is disclosed in JP-A 11-135042. Graphite may be made into a column shape, and a plurality of column shaped graphite structures may be disposed in substantially the same direction with each other, thereby to form carbon nanotube.
However, carbon nanotubes disclosed in Carbon Nanotube Field Emitter, authored by Yahachi Saito and published in the Journal of the Surface Science Society of Japan; JP-A 2000-090813; JP-A 11-135042, JP-A 2001-250496, and JP-A 2001-266780 are structured so as to have an extremely large aspect ratio (i.e., particle length/particle diameter) with a diameter of about 0.4 to 50 nm. Electrical discharge occurs initially in portions, from within a large number of carbon nanotube assemblies, at which the discharge voltage is lower. A large current then flows in localized portions, after which discharge occurs in other portions. A problem is presented in that portions that experience large localized currents degrade rapidly, increasing the likelihood of the electrical current becoming unstable, and shortening the lifespan.
The electron-emitting element disclosed in JP-A 2000-090813, i.e., the electron-emitting element in which graphite is used, is intended for use as a component of an image-displaying apparatus, not in X-ray tubes. Although this electron-emitting element has a large electrical conductivity and a low work function, making it suitable as an electron-emitting electrode, a problem has been presented in that shaping is difficult and the shape during use is not stable.
The electron-emitting element disclosed in JP-A 2000-156148; i.e., the electron-emitting element in which an end surface, formed by cutting a graphite block, a carbon rod, a carbon film, or carbon fiber in a direction perpendicular to the direction in which the carbon hexagonal planes are layered, forms the electron-emitting surface; is intended for use as a component for electron beam-utilizing instruments such as image-displaying devices, and not for X-ray tubes. In this electron-emitting element, the crystal axes of the plurality of layers of carbon hexagonal planes, namely the a-axis, the b-axis, and the c-axis, coincide with each other between each of the layers, as shown in FIG. 2 of the reference. Therefore, a problem has been presented in that the amount of electron emission from the electron-emitting surface is low.